1. Field
The present invention relates to a pigment based on bismuth vanadate, the behavior of which with respect to heat is greatly improved.
The invention also relates to the process for producing this pigment and to the application of the latter, in particular in the coloring of plastics, paints or lacquers.
2. Related Art
Bismuth vanadate is used, inter alia, as a yellow inorganic pigment. It is employed in coloring plastics and paints, also as a catalyst in the oxidation of olefins, as a contrast-enhancing pigment for television screens and as a reversible temperature indicator. It is a well known chemical compound (DE 422 947, U.S. Pat. Nos. 3,843,554, 4,115,142, JP 62277485 and DE 19733584).
Pure or modified pigments based on bismuth vanadate are nontoxic, have a vivid yellow color, have a high coloring strength and have excellent opaqueness, they are readily dispersed in their medium of use. However, their resistance, especially towards heat, is insufficient; this restricts their use in plastics or in formulations prepared or machined under hot conditions.
A large number of materials are colored with pigments: paints, lacquers, inks or plastics. In particular, compounds treated at high temperature, such as polyethylene, polystyrene, polycarbonate, polypropylene or Acrylnitril-Butadiene-Styrene (ABS-resin), are obtained by extrusion or injection molding of the colored mass softened with the heat. The use of a fairly high temperature results in an undesired change in the starting color (generally a darkening).
Numerous known processes attempt to improve the stability of pigments to light, to heat or to chemicals. These stabilization processes consist of a coating, a doping or a modification of the pigment crystal.
Patent Application EP 239 526 discloses a process for coating with a layer based on aluminum, on titanium, on cerium, on antimony, on zirconium, on silicon, on phosphorus or on zinc, in a proportion of 2 to 40%. According to Patent Application DE 4 037 878, the resistance of pigments to acids and especially to hydrochloric acid can be improved by coating with phosphates of metals, such as calcium, magnesium, aluminum, zinc, zirconium or titanium, in a proportion of 2 to 20%. Mentioned is made, in Patent Application WO 9211205, for pigments based on bismuth silicovanadate and/or phosphovanadate, of coatings of inorganic oxide, hydrate, silicate, phosphate and carbonate.
According to U.S. Pat. No. 4,063,956, the resistance to heat of thermoplastics and the resistance to chemicals is increased by coating with an inorganic precoat and subsequently with a thick layer of silicon oxide; thus, first a layer of hydrated oxides of aluminum, of silicon, of titanium, of boron, of tantalum, of molybdenum, of zinc, of manganese, of tin, of hafnium, of thorium, of niobium, of barium, of strontium, of nickel and/or of antimony is applied, with a preference for aluminum, silicon, titanium and boron. This precoat serves to increase the adhesion of the second layer, composed of amorphous and dense silicon oxide; the silica layer acts mainly to improve the resistance to heat and to acids.
A calcined pigment can also be coated in a bead mill; a first layer of silica is deposited in a basic medium and subsequently a second layer of silica is deposited, optionally in the presence of magnesium fluosilicate, of silanes or of an emulsion formed of polyethylene waxes (EP 271 813).
According to Patent Application DE 3,135,281, coating can also be carried out with layers of zirconium and silicon oxides in order to improve the resistance to light and resistance to beat in U.S. Pat. No. 4,115,142, the resistance of pigments is improved by coating with a layer of silica or of aluminum phosphate.
In Patent EP 723 998, the stability of bismuth vanadate pigments is improved by coating with compounds based on aluminum and zinc phosphates or on silicon and aluminum oxides.
In general, it may be said that any coating system recommended for improving pure or modified pigments based on bismuth vanadate resembles the well known and proven processes applied to various inorganic pigments: for example, titanium dioxides, chromium yellows, molybdenum oranges or iron oxides.
It is well known that powdered pigments, dried at more than 100xc2x0 C., exhibit a less reactive surface due to the loss of hydroxyl groups. In point of fact, it is precisely these OH groups which Tnake possible good attachment of the coating layers. Consequently, it is difficult to obtain, by coating, sufficiently reliable stabilization of the pigments, in particular those based on bismuth vanadate, which allows them to be used in plastics or paints and resins treated at high temperatures.
It is the same with doping processes. Thus, according to U.S. Pat. No. 4,026,722, a pigment based on bismuth vanadate possessing improved resistance is obtained by virtue of crystallization with silicon and aluminum oxides. The amounts used are very large and even exceed the amount of bismuth vanadate.
According to Patent GB 2,034,342, mixtures of bismuth phosphate, of vanadium pentoxide and of zinc, calcium, barium and magnesium oxides can be calcined.
Patent U.S. Pat. No. 4,781,761 discloses stabilization of a pigment based on titanium, oxide by a first layer of amorphous and dense silica, doped with 10% of boron oxide, and by a second layer of aluminum oxide (2 to 8%).
According to Patent Application DE 3,926,870, the phototropism of pigments based on bismuth vanadate is reduced by providing them with a fine layer of trivalent iron oxide or hydrate (0.001 to 2%).
According to Patent Application DE 3,906,670, the resistance to heat of preparations comprising inorganic and organic pigments can be increased by doping with boron (3 to 10% of boric acid); the sole boron derivative with a truly effective action is boric acid; sodium tetraborate (borax) is not suitable, moreover.
In Patents DE 3,136,279 and DE 3,409,722, a mixture of zinc and iron oxides is treated, before calcination, with boron compounds (H3BO3, BPO4) in a proportion of 0.1 to 1% or with compounds forming alumina phosphates (Al2O3, P2O5, and the like), in order to obtain highly crystalline pigments with a very low content of chloride.
It was already known tat chemical compounds based on aluminum, antimony, ammonium and zinc hydroxides, oxides, phosphates or borates can render organic plastics more resistant to heat and can even slow down their combustion (U.S. Pat. No. 5,248,337, BE 769,799 and GB 2,262,518). The addition of these products to the pigment itself and not to the thermoplastic makes it possible to improve the resistance to heat to a slight extent (Patent EP 370 082).
The preparation of masterbatches with pigments treated with boric acid is generally difficult (undispersed residues, nonhomogeneity, and the like). However, the compound H3BO3 is necessary in order to obtain a degree of resistance to heat. These processes do not make it possible to obtain pigments based on bismuth vanadate which are simultaneously easy to use and highly resistant to heat (300xc2x0 C.).
The present invention is targeted essentially at obtaining bismuth-based pigments which correspond to high resistance requirements, in particular to excellent resistance to heat.
Another aim of the present invention is targeted at providing a bismuth-based pigment which can be easily used in materials subjected to high temperatures.
An additional aim of the present invention consists in providing a process which makes possible reliable and efficient attachment of the coating layers to a bismuth-based pigment crystal.
The invention relates to a pigment based on bismuth vanadate which exhibits an improved resistance to heat, characterized in that it is coated with several layers of inorganic compounds m several successive stages, the last of these layers being based on a zinc and boron compound.
Preferably, the pigment is precoated with at least one first layer of inorganic compounds selected from the group consisting of hydrates, oxides, silicates, carbonates, phosphates or borates of elements selected from the group consisting of titanium, aluminum, zinc, antimony, silicon, boron, calcium, zirconium, niobium and rare earth metals, in particular cerium, lanthanum, neodymium, praseodymium, samarium or yttrium, or a mixture of these.
The pigment preferably comprises a first layer of one or more deposits of oxides, hydrates, carbonates and/or phosphates of elements selected from the group consisting of titanium, zirconium, aluminum, calcium and rare earth metals.
Secondly, one or more layers of a mixture of silicon oxide and/or hydrate is/are deposited on the pigment provided with a first layer, according to well known processes, for example precipitation under hot conditions of an alkali metal silicate solution by decreasing the pH by programmed addition of a dilute inorganic acid Finally, a third layer, which can be based on inorganic compounds formed of zinc, magnesium, aluminum and/or boron borates, oxides or hydrates, is deposited on the pigment provided with these first two layers; preferably, an inorganic compound based on zinc and on boron is deposited.
Generally, the coating of the pigment with these successive deposits of inorganic compounds represents from 0.1% to 40% of the pigment.
Additional treatments can optionally also be applied to the pigment thus stabilized. It is possible to render the pigment less dusty by treating it with dry or pasty anticaking agents: for example polyalcohols, polyfluoroethylene, aromatic acid esters, silicone and other surfactants and emulsifiers; they can also be passed into presses and granulators. It is also possible to use agents which improve the appearance and the processing, such as alcohols or polyalcohols with long aliphatic chains, fatty amines, resins and various types of anionic, cationic or nonionic surface-active agents.
A pigment based on bismuth vanadate, precoated with a first layer based on aluminum, on phosphorus, on titanium, on calcium or on rare earth metals in a proportion of 1 to 5% and then coated with a second layer based on silicon in a proportion of 5 to 25%, exhibits a degree of resistance to heat which can be further improved to a slight extent by addition of boric acid in a proportion of 1 to 10%. A pigment is then obtained with, for example, a resistance to heat in polyethylene of about 250xc2x0 C. (xcex94Exc2x0=less than or equal to 3.0).
Surpsingly and unexpectedly, the resistance to heat of a pigment based on bismuth vanadate is significantly improved further, provided that an additional layer based on 1 to 10% of boron and zinc compounds, which theoretically form a zinc borate hydrate, is precipitated on the layer based on silicon oxide.
According to the scientific literature, zinc borate hydrates are obtained by reaction in an aqueous medium of soluble zinc salts and of alkali metal borates or boric acid according to a chemical reaction of the type:
2ZnO+2H4SO4xe2x86x922ZnSO4+2H2O
2ZnSO4+2Na2B4O7xe2x86x922ZnO.3B4O2.xH2O+2Na2SO4+2H2BO3
(The water of crystallization of the reactants is not taken into account). If the reaction temperature is increased, the content of water of hydration (x) decreases: see, for example, U.S. Pat. Nos. 2,405,366, 3,126,352 and 3,649,172 and WO 9310045. Teachings on the subject of the preparation, properties and use of zinc borates are mentioned in particular in:
Mellor""s Comprehensive Treatise on Theoretical Chemistryxe2x80x94Supplement, Vol. V, Par A, 572 . . . (1980) Uhlmann""s Encyclopedia of Industrial Chemistry, Vol A4, 276 . . . (1985)
H. A. Lehman et al., Ueber wasserhaltige Zinkborate [On zinc borate hydrates], Z. for anorg. und allg. Chem., Vol. 354, No. 1, 37-43 (1967)
F. Bellingham, Zinc borate in intumescent paints, Polym. Paint Col. J., Vol. 182, No. 4319-627-8 (1992).
A resistance to heat in polyethylene of approximately 300xc2x0 C. (xcex94Exc2x0=less than or equal to 3.0) is then obtained, for example. Such a resistance is never obtained with other compounds based on borates of other metals (Na, Al, Mg, Ba, and the like). The addition of zinc borate to the masterbatch, instead of the precipitation on the pigment, does not give a resistance of greater than 250-260xc2x0 C.
This special coating with a final layer of a compound based on zinc borate provides a bismuth vanadate pigment which is furthermore very easily incorporated in colored plastic masterbatches. The ease of preparation of a masterbatch is thus combined with a very high resistance to heat; this is never obtained with other processes, such as the application of layers of boron oxide, of antimony oxide, of aluminum oxide or of magnesium or boron compounds.
Another obvious advantage of the ease of dispersion of the pigment obtained is the absence of grains in the pigmented formulations.
The invention also relates to the process for producing a pigment based on bismuth vanadate, according to which the coating is formed at the surface of the pigment crystal, very finely reslurried in an aqueous medium under hot conditions, and comprises at least one inorganic compound. A final aspect of the invention relates to the application of the pigment according to the invention in coloring plastics, industrial coatings, resins and lacquers produced or machined at high temperature.
The invention relates to a pigment based on bismuth vanadate which is very substantially improved by the coating of highly specific inorganic compounds. Unexpectedly, the pigments thus treated exhibit excellent resistance. This degree of resistance is never obtained with the processes of the prior art.
The pigment is treated in the following way, in three stages:
1st stage: the pigment is coated with a stabilizing coating based on oxides or hydrates of titanium, of mare earth metals, of aluminum, of phosphorus or of calcium, according to a proven and known technique.
2nd stage: the pigment is resuspended in an aqueous medium with a source of silica, which is deposited at high temperature (60-100xc2x0 C.) by addition of acid, according to a proven and known technique.
3rd stage: finally, a compound based on zinc borate is precipitated from zinc and boron salts.
This third stage is determining for the production of a bismuth vanadate pigment possessing high resistance to heat. If it is omitted, if it is used without the other two stages or if other chemical compounds are used, the expected effect is not obtained. Furthermore, it is necessary to follow the order indicated for the three stages and none thereof may be omitted: in that case, an effect is obtained which is greater than that which might be expected from each of the stages taken separately.
Thus, aluminum oxides or hydrates might be used (U.S. Pat. No. 4,063,956) or silica might be precipitated with boron oxide or hydrate (U.S. Pat. No. 4,784,761) but the results obtained are markedly worse. A physical mixture of a zinc borate and of a bismuth vanadate pigment treated in a known way with silica (U.S. Pat. No. 3,649,172) might also be prepared in proportions 10:90 to 1:99. The degree of resistance to heat obtained according to the process of the invention is never achieved in this case.
Use may be made, as boron source, of alkali metal metaborates and tetraborates, mainly sodium and potassium tetraborates Na2B4O7.10H2O, K2B4O7.4H2O or Na2B4O7.5H2O, and boric acid H3BO3.
Use may be made, as zinc source, of soluble zinc salts, for example acetate (CH3COO)2ZN.2H2O, chloride ZnCl2, nitrate Zn(NO3)2.6H2O, sulfate ZnSO4.7H2O as well as oxide ZnO, provided that it is dissolved before hand in the appropriate amount of acid (acetic, nitric, hydrochloric or sulfuric).